U.S. patent number 5,422,461 [Application Number 07/990,530] was granted by the patent office on 1995-06-06 for control device and safety circuit for heating pads with ptc heater.
This patent grant is currently assigned to Micro Weiss Electronics, Inc.. Invention is credited to K. Y. Lin, John Weiss.
United States Patent |
5,422,461 |
Weiss , et al. |
June 6, 1995 |
Control device and safety circuit for heating pads with PTC
heater
Abstract
Heating pads using positive temperature coefficient (PTC)
resistance material are subject to fire risk if one of the
conductor wires between which the PTC material extends should break
and produce an electric arc. Protection by fuse and a fuse-blowing
circuit responsive to fire detection must allow for an immense
inrush of current when the cold pad is turned on. A heat setting
control using a microprocessor can reduce the fuse rating by
chopping the a.c. heating current for a short start-up period
following with full-on feed until the heat setting is reached. The
presence of a microprocessor allows response to a safety circuit
that detects a break in a heater feed or return conductor before
much excess heat develops, so that the microprocessor can turn off
the heater switch. That response is so quick that it can be
confirmed by repeated detection after very short pauses before the
heater switch (a triac) is turned off. The safety circuit producing
the fault detection signal may be external to the microprocessor
chip, or most of it can be built into the microprocessor chip,
which then receives two inputs from a smaller circuit connected to
the heating pad. A second triac can be used to shut off the heater
if the heater switch malfunctions by locking in its "on"
position.
Inventors: |
Weiss; John (Amityville,
NY), Lin; K. Y. (Taipei, TW) |
Assignee: |
Micro Weiss Electronics, Inc.
(West Babylon, NY)
|
Family
ID: |
25536253 |
Appl.
No.: |
07/990,530 |
Filed: |
December 15, 1992 |
Current U.S.
Class: |
219/501; 219/212;
219/492; 219/504; 219/505; 219/508; 323/235 |
Current CPC
Class: |
G05D
23/2401 (20130101) |
Current International
Class: |
G05D
23/20 (20060101); G05D 23/24 (20060101); H05B
001/02 () |
Field of
Search: |
;219/212,497,492,494,501,505,504,508-510,481
;323/235,236,901,319,908 ;307/117 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Paschall; Mark H.
Attorney, Agent or Firm: Frishauf, Holtz, Goodman &
Woodward
Claims
We claim:
1. A safety circuit for an electric alternating-current heater,
said heater having a heating element provided by a web of PTC
electroresistive material extending between first and second heater
feed conductors, said first heater feed conductor (1) being
connectable through a protecting fuse (5) to an ungrounded pole of
a source of alternating electric current and said second heater
feed conductor being connectable through a heater switch (T1) to a
grounded pole of said source of alternating electric current, said
first and second heater feed conductors, at their respective ends
remote from said fuse and from said heater switch, being
respectively connected to first and second safety link return
conductors leading respectively to first (3) and second (4) inputs
of said safety circuit, said heater switch being a triac having a
control input, connected to an output of an integrated circuit (IC)
unit, for duty cycle time division control of said triac through
said control input of said triac in a duty cycle range varying from
at most 25% duty to 100% in response to heat settings and in
response to an output of said safety circuit connected to a control
input of said integrated circuit unit, said integrated circuit unit
being supplied with d.c. power at a reference voltage by d.c. power
supply means connected to said source of alternating electric
current, said integrated circuit unit also having manual control
means for controlling said heater, said safety circuit
comprising:
a first resistive voltage divider (R1, R2) having a tap connection
and a greater and a smaller resistance respectively on opposite
sides of said tap connection and connected between the end of said
second heater feed conductor (2) which end is connected to said
heater switch (T1) and the end of said first safety link return
conductor which is remote from its connection to said first heater
feed conductor;
a second resistive voltage divider having a tap connection and a
greater and a smaller resistance respectively on opposite sides of
said tap connection and connected between the end of said first
heater feed conductor (1) which end is connected to said fuse (5)
and the end of said second safety link conductor (4) which end is
remote from its connection to said second heater feed conductor
(2), the greater of said resistances of each of said first and
second voltage dividers being connected respectively to said first
safety link return conductor and to said first heater feed
conductor,
the ratio of said greater to said smaller resistance being greater
for said second voltage divider than for said first voltage
divider;
a first rectifier diode (D1) having a first electrode connected to
said tap of said first voltage divider and a second electrode
connected to a first network comprising a first capacitor (Cl)
shunted by a resistor (R5) leading to ground potential, said first
network having a first predetermined time constant;
a second rectifier diode (D2) having a first electrode connected to
said tap of said second voltage divider and a second electrode
connected to a second network comprising a second capacitor (C2)
shunted by another resistor (R6) leading to ground potential, said
second network having a second time constant;
said connection of said first rectifier (D1) to said first
capacitor (C1) also being connected, through a third diode (D3),
poled oppositely to said first diode (D1) in a series connection
therewith interposed between said first diode and said control
input of said integrated circuit unit;
a semi-conductor inverting amplifier stage having a first main path
electrode connected to said reference voltage, having a control
electrode connected, through an input resistor (R8) to said second
diode (D2) where said second diode is connected to said second
capacitor (C2), for blocking conduction between main path
electrodes of said amplifier stage by a signal rectified by said
second diode (D2) and having a second main path electrode connected
both to a load resistor (RIO) leading to ground potential and,
through a fourth diode (D4), to said control input (9) of said
integrated circuit (IC) unit (14), the polarity of said fourth
diode (D4) being opposite to that of said third diode (D3) when
said third and fourth diodes are considered as being in series
through their common connection and being the same as the polarity
of said third diode when said third and fourth diodes are
considered as respectively belonging to parallel paths to ground
through respective resistances (R5, R10);
wherein said first and second networks serve to make a time that
passes after initial power up of said heater before said integrated
circuit unit can respond to a fault substantially equal to a time
necessary for both the safety circuit and the integrated circuit
(IC) unit to react to a fault.
2. The safety circuit of claim 1 wherein a predetermined minimum
value of said duty cycle is made effective by said integrated
circuit unit at said control input of said triac in initial startup
to reduce the initial average current for a predetermined initial
time interval so that said fuse (5) can be selected without regard
to inrush current values that might otherwise occur and wherein
said triac is fired in consecutive cycles of said alternating
electric current of said alternating electric current source
immediately after each null voltage transition of said consecutive
alternating electric current cycles.
3. The safety circuit of claim 1 wherein said first network and
said second network each have a time constant not less than 0.04
sec. and not greater than 0.06 sec.
4. The safety circuit of claim 2 wherein said first network and
said second network each have a time constant not less than 0.04
sec. and not greater than 0.06 sec.
5. The safety circuit of claim 1, wherein said first voltage
divider has a ratio between 8% and 12% said second voltage divider
has a ratio between 0.3% and 0.7% and wherein the overall
resistance of said second voltage divider is at least 50% greater
than the overall resistance of said first voltage divider.
6. The safety circuit of claim 2, wherein said first voltage
divider has a ratio between 8% and 12%, said second voltage divider
has a ratio between 0.3% and 0.7% and wherein the overall
resistance of said second voltage divider is at least 50% greater
than the overall resistance of said first voltage divider.
7. A safety-assuring control device for an electric
alternating-current heater, said heater having a heating element
provided by a web of PTC electroresistive material extending
between first and second heater feed conductors, said first heater
feed conductor (1) being connected to a protecting fuse (5) and
connectable therethrough to an ungrounded pole of a source of
alternating electric current and said second heater feed conductor
(2) being connected to an electrically controlled heater switch
(T1) and connectable therethrough to a grounded pole of said source
of alternating electric current, said first and second heater feed
conductors, at respective ends remote from said fuse and from said
heater switch, being respectively connected to first and second
safety link return conductors which lead respectively towards first
and second connections to said control device, said control device
comprising an integrated circuit unit (14) having means for
cyclically varying the on time of said heater switch in consecutive
equal periods of a constant major fraction of a minute from an on
time of a few second stepwise to a continuous on time, a safety
circuit connected to said first and second safety link return
conductors and to an input (9) of said integrated circuit unit (14)
and a source of a direct current supplied at a steady potential
more than 4 volts and less than 7 volts from ground potential,
connected to an input of said integrated circuit unit (14) and to
said safety circuit,
said electrically controlled heater switch is a triac having
control connections means (12);
said integrated circuit unit (14) has a safety circuit input, an
input connected with said source of direct current, as well as
least one heat setting input (15, 16, 17) connected to heat setting
means, a grounding connection, an input (20) for voltage of said
electric alternating current and an output connected to said
control connection means (12) of said triac and is programmed for
control of said means for cyclically varying the on time of said
heater switch in response to said heat setting means and to said
safety circuit and for otherwise interrupting or shutting off said
heater switch in response to occasional input from said safety
circuit,
a first resistive voltage divider (R1,R2) having a tap connection
and a greater and a smaller resistance respectively on opposite
sides of said tap connection and connected between the end of said
second heater feed conductor (2) which end is connected to said
heater switch (T1) and the end of said first safety link return
conductor which is remote from its connection to said first heater
feed conductor;
a second resistive voltage divider having a tap connection and a
greater and a smaller resistance respectively on opposite sides of
said tap connection and connected between the end of said first
heater feed conductor (1) which end is connected to said fuse (5)
and the end of said second safety link conductor (4) which end is
remote from its connection to said second heater feed conductor
(2), the greater of said resistances of each of said first and
second voltage dividers being connected respectively to said first
safety link return conductor and to said first heater feed
conductor,
the ratio of said greater to said smaller resistance being greater
for said second voltage divider than for said first voltage
divider;
a first rectifier diode (D1) having a first electrode connected to
said tap of said first voltage divider and a second electrode
connected to a first network comprising a first capacitor (C1)
shunted by a resistor (R5) leading to ground potential, said first
network having a first predetermined time constant;
a second rectifier diode (D2) having a first electrode connected to
said tap of said second voltage divider and a second electrode
connected to a second network comprising a second capacitor (C2)
shunted by another resistor (R6) leading to ground potential, said
second network having a second time constant;
said connection of said first rectifier (D1) to said first
capacitor (Cl) also being connected, through a third diode (D3),
poled oppositely to said first diode (D1) in a series connection
therewith interposed between said first diode and said control
input of said integrated circuit (IC) unit;
a semi-conductor inverting amplifier stage having a first main path
electrode connected to said reference voltage, having a control
electrode connected, through an input resistor (R8) to said second
diode (D2) where said second diode is connected to said second
capacitor (C2), for blocking conduction between main path
electrodes of said amplifier stage by a signal rectified by said
second diode (D2) and having a second main path electrode connected
both to a load resistor (R16) leading to ground potential and,
through a fourth diode (D4), to said control input of said
integrated circuit unit, the polarity of said fourth diode (D4)
being opposite to that of said third diode (D3) when said third and
fourth diodes are considered as being in series through their
common connection and being the same as the polarity of said third
diode when said third and fourth diodes are considered as
respectively belonging to parallel paths to ground through
respective resistances (R5,R16);
whereby at low voltage at said input (9) of said integrated circuit
unit (14) causes a shutting off of said heater switch at least for
a predetermined period and
wherein said first and second networks serve to make a time that
passes after initial power-up of said heater before said integrated
circuit unit can respond to a fault substantially equal to a time
necessary for both the safety circuit and the integrated circuit
unit to react to a fault.
8. The safety-assuring control device of claim 7 wherein a
predetermined minimum non-zero value of said on time of said heater
switch in consecutive equal periods is made effective by said
integrated circuit unit at said control input of said triac in
initial startup to reduce the initial average current for a
predetermined initial time interval so that said fuse (5) can be
selected without regard to inrush current values that might
otherwise occur and wherein said triac is fired in consecutive
cycles of said alternating electric current of said alternating
electric current source immediately after each null voltage
transition of said consecutive alternating electric current
cycles.
9. The safety-assuring control device of claim 7, wherein said
integrated circuit unit (14) is programmed to provide a duty cycle
period not exceeding one minute in which variable on-time has a
minimum duration of at least one second and the duty cycle varies
from at most 10% to full on, and wherein said integrated circuit
unit is able to detect a voltage designating a fault reported by
said safety circuit in substantially less than 0.2 sec., and
wherein said integrated circuit unit is programmed from an initial
detection of said voltage to interrupt the pulses necessary for
keeping the triac heater switch in its on condition for a major
fraction of a second instead of turning the triac off entirely, and
wherein, after at least one interruption for a major fraction of a
second, the detection of another fault reported by said safety
circuit results in turning off said triac heater switch
indefinitely.
10. The safety-assuring control device of claim 8, wherein said
integrated circuit unit (14) is programmed to provide a duty cycle
period not exceeding one minute in which variable on-time has a
minimum of at least one second duration and the duty cycle varies
from at most 10% to full on, and wherein said integrated circuit
unit is able to detect a voltage designating a fault reported by
said safety circuit in substantially less than 0.2 sec., and
wherein said integrated circuit unit is programmed from an initial
detection of said voltage to interrupt the pulses necessary for
keeping the triac heater switch in its on condition for a major
fraction of a second instead of turning the triac off entirely, and
wherein, after at least one interruption for a major fraction of a
second, the detection of another fault reported by said safety
circuit results in turning off said triac heater switch
indefinitely.
11. The safety-assuring control device of claim 7 wherein said
integrated circuit unit is programmed to respond, after a triac
turn off for a major fraction of a second in response to an initial
fault detection, by again reading a signal at said safety circuit
input (9) of said integrated circuit unit and if a fault is again
detected, the turn on of the triac is again blocked and an LCD
display is switched into a blinking mode.
12. The safety-assuring control device of claim 8 wherein said
integrated circuit unit is programmed to respond, after a triac
turn off for a major fraction of a second in response to an initial
fault detection, by again reading a signal at said safety circuit
input (9) of said integrated circuit unit and if a fault is again
detected, the turn on of the triac is again blocked and an LCD
display is switched into a blinking mode.
13. The safety-assuring control device of claim 7, wherein said
d.c. power supply means provides d.c. power at an electro-positive
reference voltage with respect to ground potential.
14. The safety-assuring control device of claim 8 wherein said d.c.
power supply means provides d.c. power at an electro-positive
reference voltage with respect to ground potential.
15. The safety-assuring control device of claim 7, wherein said
heater switch comprises first and second triacs (T1,T2) each having
a control input connected to an individual output of said
integrated circuit unit, said first triac (T1) being connected to
said second heater feed conductor and to said second triac (T2) and
said second triac (T2) being connected to said grounded pole of
said source of alternating electric current, and wherein said
integrated circuit unit is programmed for switching on both of said
triacs simultaneously through their respective control inputs,
whereby if a triac fails to respond to a turn off condition at its
control input, the turn off is nevertheless accomplished by the
other triac.
16. The safety-assuring control device of claim 8, wherein said
heater switch comprises first and second triacs (T1,T2) each having
a control input connected to an individual output of said
integrated circuit unit, said first triac (T1) being connected to
said second heater feed conductor and to said second triac (T2) and
said second triac (T2) being connected to said grounded pole of
said source of alternating electric current, and wherein said
integrated circuit unit is programmed for switching on both of said
triacs simultaneously through their respective control inputs,
whereby if a triac fails to respond to a turn off condition at its
control input, the turn off is nevertheless accomplished by the
other triac.
17. A safety-assuring control device for an electric
alternating-current heater, said heater having a heating element
provided by a web of PTC electroresistive material extending
between first and second heater feed conductors, said first heater
feed conductor (1) being connected to a protecting fuse (5) and
connectable therethrough to an ungrounded pole of a source of
alternating electric current and said second heater feed conductor
being connected to an electrically controlled heater switch (T1)
and connectable therethrough to a grounded pole of said source of
alternating electric current, said first and second heater feed
conductors, at respective ends remote from said fuse and from said
heater switch, being respectively connected to first and second
safety link return conductors which lead respectively towards
connections to said control device, said control device comprising
an integrated circuit unit having a read-only memory, a program
counter, an arithmetic logic unit and, a random access memory, and
a data bus interconnecting at least said arithmetic logic unit,
said program counter, said random access memory and a time counter,
said read-only memory being connected to said program counter, said
arithmetic logic unit and said random access memory, wherein:
said electrically controlled heater switch is a triac having
control connections means (12);
said integrated circuit unit (27) has first and second safety
circuit inputs (22,23) and an input connected with a source of
direct current supplied at steady potential more than 4 volts and
less than 7 volts from ground potential, as well as least one heat
setting input (15,16,17), a grounding connection, an input (20) for
voltage of said electric alternating current and an output
connected to said control connection means (12) of said triac;
said second safety link return conductor, at its end adjacent to
said control device, is clamped to ground to ground potential in a
first polarity and to said steady d.c. potential in a second
polarity, opposite to said first polarity, by respective diodes
(D9,D10), is connected to a first current limiting resistor (12)
leading to the connection of said fuse with said first heater feed
conductor and, is connected to said second safety circuit input
(23) of said integrated circuit unit (27); and
said first safety link return conductor, at its end adjacent to
said control device, is connected, at least after an applied
initial voltage drop exceeding 50 volts, to a second current
limiting resistor (10) leading to a junction (25) which, in
addition to being connected to said second current limiting
resistor, is connected to a third current limiting resistor (11)
which leads to ground potential, said junction being clamped to
ground in said first polarity and to said steady d.c. potential in
said second polarity by respective diodes (D6,D8) said junction
being connected to said first safety circuit input (23) of said
integrated circuit unit through a diode for selecting half cycles
of alternating voltage corresponding to said alternating electric
which are of a predetermined polarity;
said input (20) of said integrated circuit unit (27) for voltage of
said electric alternating current being clamped by a diode (D5) to
said steady d.c. potential and connected through a fourth current
limiting resistor (R9) to said first heater feed conductor at or
near its connection to said fuse (5), and
said integrated circuit unit being programmed by its read-only
memory to enable said triac to conduct alternating current
continuously or periodically so long as a.c. power frequency pulses
going from ground potential to approximately said potential of said
source of direct current are supplied to said first safety circuit
input (22) of said integrated circuit unit while the potential at
said second safety circuit input (23) of said integrated circuit
unit remains within a predetermined voltage, less than one volt,
from ground potential and to disable said triac for at least half a
second when a.c. power frequency pulses going to approximately said
potential of said source of direct current are supplied to said
second safety circuit input (23) of said integrated circuit unit
and likewise when the potential at said first safety circuit input
(22) remains within said predetermined voltage, less than one volt,
from ground potential.
18. A safety-assuring control device for an electric
alternating-current heater, said heater having a heating element
provided by a web of PTC electroresistive material extending
between first and second heater feed conductors, said first heater
feed conductor (1) being connected to a protecting fuse (5) and
connectable therethrough to an ungrounded pole of a source of
alternating electric current and said second heater feed conductor
being connected to an electrically controlled heater switch (T1)
and connectable therethrough to a grounded pole of said source of
alternating electric current, said first and second heater feed
conductors, at respective ends remote from said fuse and from said
heater switch, being respectively connected to first and second
safety link return conductors which lead respectively towards
connections to said control device, said control device comprising
an integrated circuit unit having a read-only memory, a program
counter, an arithmetic logic unit and, a random access memory, and
a data bus interconnecting at least said arithmetic logic unit,
said program counter, said random access memory and a time counter,
said read-only memory being connected to said program counter, said
arithmetic logic unit and said random access memory, wherein:
said integrated circuit unit (34) has first and second safety
circuit inputs (22,23) and an input connected with a source of
direct current supplied at steady potential more than 4 volts and
less than 7 volts form ground potential, as well as least one heat
setting input (15,16,17), a grounding connection, an input (20) for
voltage of said electric alternating current and an output
connected to said control connection means (12) of said triac;
said second safety link return conductor, at its end adjacent to
said control device, is connected to said second safety circuit
input (23) of said integrated circuit unit (27) and is connected to
the tap connection a resistive voltage divider (R13,R14) connected
from ground potential to said d.c. potential so as to put said tap
connection at no more than 1 volt from ground potential; and
said first safety link return conductor, at its end adjacent to
said control device, is connected, at least after an applied
initial voltage drop exceeding 50 volts, to a first current
limiting resistor (10) leading to a junction (25) which, in
addition to being connected to said first current limiting
resistor, is connected to said first safety circuit input (23) of
said integrated circuit unit and is connected to a second current
limiting resistor (11) which leads to ground potential, said
junction being clamped to ground potential in a first polarity and
to said steady d.c. potential in a second polarity, opposite to
said first polarity, by respective diodes (D6,D7);
said input (20) of said integrated circuit unit (27) for voltage of
said electric alternating current being clamped by a diode (D5) to
said steady d.c. potential and connected through a third current
limiting resistor (R9) to said first heater feed conductor at or
near its connection to said fuse (5), and
said integrated circuit unit being programmed by its read-only
memory to enable said triac to conduct alternating current
continuously or periodically so long as a.c. power frequency pulses
going from ground potential to approximately said potential of said
source of direct current are supplied to said first safety circuit
input (22) of said integrated circuit unit while the potential at
said second safety circuit input (23) of said integrated circuit
unit remains within a predetermined voltage, less than one volt,
from ground potential and to disable said triac for at least half a
second when a.c. power frequency pulses going to approximately said
potential of said source of direct current are supplied to said
second safety circuit input (23) of said integrated circuit unit
and likewise when the potential at said first safety circuit input
(22) remains within said predetermined voltage, less than one volt,
from ground potential.
19. The safety-assuring control device of claim 17, wherein a
predetermined minimum value of said duty cycle is made effective by
said integrated circuit unit at said control input of said triac in
initial startup to reduce the initial average current for a
predetermined initial time interval so that said fuse (5) can be
selected without regard to inrush current values that might
otherwise occur and wherein said triac is fired in consecutive
cycles of said alternating electric current of said alternating
electric current source immediately after each null voltage
transition of said consecutive alternating electric current
cycles.
20. The safety-assuring control device of claim 18, wherein a
predetermined minimum value of said duty cycle is made effective by
said integrated circuit unit at said control input of said triac in
initial startup to reduce the initial average current for a
predetermined initial time interval so that said fuse (5) can be
selected without regard to inrush current values that might
otherwise occur and wherein said triac is fired in consecutive
cycles of said alternating electric current of said alternating
electric current source immediately after each null voltage
transition of said consecutive alternating electric current
cycles.
21. The safety-assuring control device of claim 17, wherein said
integrated circuit unit (27) is programmed to provide a duty cycle
not exceeding one minute in which variable on-time has a minimum
duration of at least one second and the duty cycle varies from at
most 10% to full on, and wherein said integrated circuit unit is
able to detect a voltage designating a fault reported by said
safety circuit in substantially less than 0.2 sec., and wherein
said integrated circuit unit is programmed from an initial
detection of said voltage to interrupt the pulses necessary for
keeping the triac heater switch in its on condition for a major
fraction of a second instead of turning the triac off entirely, and
wherein, after at least one interruption for a major fraction of a
second, the detection of another fault reported by said safety
circuit results in turning off said triac heater switch
indefinitely.
22. The safety-assuring control device of claim 18, wherein said
integrated circuit unit (27) is programmed to provide a duty cycle
not exceeding one minute in which variable on-time has a minimum
duration of at least one second and the duty cycle varies from at
most 10% to full on, and wherein said integrated circuit unit is
able to detect a voltage designating a fault reported by said
safety circuit in substantially less than 0.2 sec., and wherein
said integrated circuit unit is programmed from an initial
detection of said voltage to interrupt the pulses necessary for
keeping the triac heater switch in its on condition for a major
fraction of a second instead of turning the triac off entirely, and
wherein, after at least one interruption for a major fraction of a
second, the detection of another fault reported by said safety
circuit results in turning off said triac heater switch
indefinitely.
23. The safety-assuring control device of claim 17 wherein said
integrated circuit unit is programmed to respond, after a triac
turn off for a major fraction of a second in response to an initial
fault detection, by again reading signals at safety circuit inputs
of said integrated circuit unit and if the fault is again detected,
the turn on of the triac is again blocked and an LCD display is
switched into a blinking mode.
24. The safety-assuring control device of claim 8 wherein said
integrated circuit unit is programmed to respond, after a triac
turn off for a major fraction of a second in response to an initial
fault detection, by again reading signals at safety circuit inputs
of said integrated circuit unit and if the fault is again detected,
the turn on of the triac is again blocked and an LCD display is
switched into a blinking mode.
25. The safety-assuring control device of claim 17 wherein said
integrated circuit unit includes an independently running counter
connected for resetting said integrated circuit unit to a
predetermined place in a program for controlling operations of said
integrated circuit unit at regular intervals.
26. The safety-assuring control device of claim 18 wherein said
integrated circuit unit includes an independently running counter
connected for resetting said integrated circuit unit to a
predetermined place in a program for controlling operations of said
integrated circuit unit at regular intervals.
27. The safety-assuring control device of claim 17, wherein said
heater switch comprises first and second triacs (T1,T2) each having
a control input connected to an individual output of said
integrated circuit unit, said first triac (T1) being connected to
said second heater feed conductor and to said second triac (T2) and
said second triac (T2) being connected to said grounded pole of
said source of alternating electric current, and wherein said
integrated circuit unit is programmed for switching on both of said
triacs simultaneously through their respective control inputs,
whereby if a triac fails to respond to a turn off condition at its
control input, the turn off is nevertheless accomplished by the
other triac.
28. The safety-assuring control device of claim 18, wherein said
heater switch comprises first and second triacs (T1,T2) each having
a control input connected to an individual output of said
integrated circuit unit, said first triac (T1) being connected to
said second heater feed conductor and to said second triac (T2) and
said second triac (T2) being connected to said grounded pole of
said source of alternating electric current, and wherein said
integrated circuit unit is programmed for switching on both of said
triacs simultaneously through their respective control inputs,
whereby if a triac fails to respond to a turn off condition at its
control input, the turn off is nevertheless accomplished by the
other triac.
29. The safety-assuring control device of claim 19, wherein said
heater switch comprises first and second triacs (T1,T2) each having
a control input connected to an individual output of said
integrated circuit unit, said first triac (T1) being connected to
said second heater feed conductor and to said second triac (T2) and
said second triac (T2) being connected to said grounded pole of
said source of alternating electric current, and wherein said
integrated circuit unit is programmed for switching on both of said
triacs simultaneously through their respective control inputs,
whereby if a triac fails to respond to a turn off condition at its
control input, the turn off is nevertheless accomplished by the
other triac.
30. The safety-assuring control device of claim 20, wherein said
heater switch comprises first and second triacs (T1,T2) each having
a control input connected to an individual output of said
integrated circuit unit, said first triac (T1) being connected to
said second heater feed conductor and to said second triac (T2) and
said second triac (T2) being connected to said grounded pole of
said source of alternating electric current, and wherein said
integrated circuit unit is programmed for switching on both of said
triacs simultaneously through their respective control inputs,
whereby if a triac fails to respond to a turn off condition at its
control input, the turn off is nevertheless accomplished by the
other triac.
31. The safety-assuring control device of claim 17, wherein said
d.c. power supply means provides d.c. power at an electro-positive
reference voltage with respect to ground potential.
32. The safety-assuring control device of claim 18, wherein said
d.c. power supply means provides d.c. power at an electro-positive
reference voltage with respect to ground potential.
33. The safety-assuring control device of claim 17, wherein said
connection of said first safety link return conductor, at its end
adjacent to said control device is through a break-over device
having a minimum breakdown turn-on voltage which is greater than 50
volts, said break-over device having a first electrode connected to
said first safety link return conductor and a second electrode
connected to said second current limiting resistor (10).
34. The safety-assuring control device of claim 18, wherein said
connection of said first safety link return conductor, at its end
adjacent to said control device is through a break-over device
having a minimum breakdown turn-on voltage which is greater than 50
volts, said break-over device having a first electrode connected to
said first safety link return conductor and a second electrode
connected to said first current limiting resistor (12).
35. The safety-assuring control device of claim 33, wherein said
break-over device comprises a bulb containing a gas capable of
being ionized by an electrical field and wherein a leak resistor
having less resistance than said second current-limiting resistor
(10) is connected between said second electrode of said break-over
device and said second safety link return conductor.
36. The safety-assuring control device of claim 34, wherein said
break-over device comprises a bulb containing a gas capable being
ionized by an electrical field and wherein a leak resistor having
less resistance than said first current-limiting resistor (12) is
connected between said second electrode of said break-over device
and said second safety link return conductor.
37. The safety-assuring control device of claim 35, wherein a
predetermined minimum value of said duty cycle is made effective by
said integrated circuit unit at said control input of said triac in
initial startup to reduce the initial average current for a
predetermined initial time interval so that said fuse (5) can be
selected without regard to inrush current values that might
otherwise occur and wherein said triac is fired in consecutive
cycles of said alternating electric current of said alternating
electric current source immediately after each null voltage
transition of said consecutive alternating electric current
cycles.
38. The safety-assuring control device of claim 36, wherein a
predetermined minimum value of said duty cycle is made effective by
said integrated circuit unit at said control input of said triac in
initial startup to reduce the initial average current for a
predetermined initial time interval so that said fuse (5) can be
selected without regard to inrush current values that might
otherwise occur and wherein said triac is fired in consecutive
cycles of said alternating electric current of said alternating
electric current source immediately after each null voltage
transition of said consecutive alternating electric current
cycles.
39. The safety-assuring control device of claim 35, wherein said
integrated circuit unit (27) is programmed to provide a duty cycle
not exceeding one minute in which variable on-time has a minimum
duration of at least one second and the duty cycle varies from at
most 10% to full on, and wherein said integrated circuit unit is
able to detect a voltage designating a fault reported by said
safety circuit in substantially less than 0.2 sec., and wherein
said integrated circuit unit is programmed from an initial
detection of said voltage to interrupt the pulses necessary for
keeping the triac heater switch in its on condition for a major
fraction of a second instead of turning the triac off entirely, and
wherein, after at least one interruption for a major fraction of a
second, the detection of another fault reported by said safety
circuit results in turning off said triac heater switch
indefinitely.
40. The safety-assuring control device of claim 36, wherein said
integrated circuit unit (27) is programmed to provide a duty cycle
not exceeding one minute in which variable on-time has a minimum
duration of at least one second and the duty cycle varies from at
most 10% to full on, and wherein said integrated circuit unit is
able to detect a voltage designating a fault reported by said
safety circuit in substantially less than 0.2 sec., and wherein
said integrated circuit unit is programmed from an initial
detection of said voltage to interrupt the pulses necessary for
keeping the triac heater switch in its on condition for a major
fraction of a second instead of turning the triac off entirely, and
wherein, after at least one interruption for a major fraction of a
second, the detection of another fault reported by said safety
circuit results in turning off said triac heater switch
indefinitely.
41. The safety-assuring control device of claim 35 wherein said
integrated circuit unit is programmed to respond, after a triac
turn off for a major fraction of a second in response to an initial
fault detection, by reading again safety circuit input signal and
if the fault is again detected, the turn on of the triac is again
blocked and an LCD display is switched into a blinking mode.
42. The safety-assuring control device of claim 36 wherein said
integrated circuit unit is programmed to respond, after a triac
turn off for a major fraction of a second in response to an initial
fault detection, by reading again safety circuit input signal and
if the fault is again detected, the turn on of the triac is again
blocked and an LCD display is switched into a blinking mode.
43. The safety-assuring control device of claim 35 wherein said
integrated circuit unit includes an independently running counter
connected for resetting said integrated circuit unit to a
predetermined place in the program of said integrated circuit unit
at regular intervals.
44. The safety-assuring control device of claim 36 wherein said
integrated circuit unit includes an independently running counter
connected for resetting said integrated circuit unit to a
predetermined place in the program of said integrated circuit unit
at regular intervals.
45. The safety-assuring control device of claim 35, wherein said
d.c. power supply means provides d.c. power at an electro-positive
reference voltage with respect to ground potential.
46. The safety-assuring control device of claim 36, wherein said
d.c. power supply means provides d.c. power at an electro-positive
reference voltage with respect to ground potential.
47. The safety-assuring control device of claim 17, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (27) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
48. The safety-assuring control device of claim 18, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (34) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
49. The safety-assuring control device of claim 35, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (27) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 m and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
50. The safety-assuring control device of claim 36, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (34) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
51. The safety-assuring control device of claim 37, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (27) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
52. The safety-assuring control device of claim 38, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (34) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
53. The safety-assuring control device of claim 39, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (27) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
54. The safety-assuring control device of claim 40, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (34) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
55. The safety-assuring control device of claim 41, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (27) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
56. The safety-assuring control device of claim 42, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (34) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
57. The safety-assuring control device of claim 43, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (27) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
58. The safety-assuring control device of claim 44, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (34) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
59. The safety-assuring control device of claim 45, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (27) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the count
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
60. The safety-assuring control device of claim 46, wherein a
second triac (T2), having a control input, is connected between the
junction of said first heater feed conductor (1) and said fuse (5)
and ground potential and has its control input connected to a
second control output of said integrated circuit unit (34) and
wherein said integrated circuit unit is programmed to count the
number of turnoffs of turn-off commands for the triac (T1) of said
electrically controlled heater switch and to compare the counter
number with a predetermined number which is at least 3 and less
than 17, and when said predetermined number of said turn-off
commands is reached, said second triac is turned on for blowing
said fuse (5) and thereby disabling the heater.
Description
BACKGROUND OF THE INVENTION
This invention is in the field of control devices and safety
circuits for electrical heating pads and the like which use
positive temperature co-efficient (PTC) materials for a heating
element. It particularly concerns safety circuits and controls for
alternating current heating pads because they usually involve
higher voltages.
Heating pads and electric blankets are appliances that, by their
nature, conduct high current electrical power in close proximity to
the user. Besides the obvious danger of electrifusion as from any
electrical appliance, a health concern exists regarding the
prolonged exposure to the electromagnetic radiation. Heaters of the
PTC type are known to be configured so as to virtually eliminate
the magnitude of the electromagnetic fields thought to be harmful.
The safe operation of the PTC heating elements is the focus of the
present invention.
PTC materials used for heating elements have the added safety of
limiting the current draw as the temperature approaches the design
limit. With this in mind a heater can be designed without the need
for an additional temperature limiting device, such as is disclosed
in Crowely U.S. Pat No. 4,271,350. Due to the nonlinear response of
temperature with current, sufficient temperature control can be
achieved by proportioning power to the heater. The only condition
that subverts the inherent safety of the PTC heating element is
when one of the conductors, in intimate contact with the PTC
material, breaks and arcing occurs. To prevent this condition from
continuing and possibly causing fire, a safety curcuit is commonly
used that detects the condition, and generates a current surge
designed to blow the power input fuse, so that the unit is thereby
disabled. Carlson, U.S. Pat No. 4,436,986, teaches the idea of
sensing voltage changes and conducting sufficient current to
disable the unit when neon bulbs exceed their breakdown voltages.
Carlson goes further and incorporates three electrodes within a
neon lamp forming a triode that breaks down at a single
predetermined voltage, thus reducing the effect of break down
voltage tolerance. Carlson uses a current limiting resistor to blow
the fuse in a predetermined period of time. It is necessary for the
current limiting resistor to be rated at a higher power than the
fuse to provide a safe open circuit. The fuse, however, must be
sized to handle currents of two to three times the continuous
current rating of the heater to accommodate the inrush associated
with the start up characteristic of the Positive Temperature
Coefficient material. The fuse is also relied upon in Carlson's
invention to deactivate the unit in all possibilities of short
circuits.
A further development that improves the safety of a PTC heating
element is taught by Clifford Stine, U.S. Pat. No. 5,081,339. Stine
reduces the possibility of breakage and improves the heat
dissipation when incorporating a PTC heating wire within a coplanar
sandwiched construction, in conjunction with the heating of a
waterbed so that the construction is also leak tight. A heat
conductive layer and the local current throttling effect of the PTC
material combine to provide the most efficient heating without
occurrence of hot spots along any part of the heating element.
Typically, an adjustable bimetalic control switch is used to
provide differing heat settings for the PTC heating. As the current
flows through the bimetalic element, it heats up causing the
element to bend due to the differential expansion of the metals
that comprise the elements. The deflection causes the contacts to
open and interrupt the current to the heater and the small
bimetalic element to cease bending. The bimetalic element then
cools down until contact is again made and the cycle repeats. The
deactivation of this type of electro mechanical control, for safety
reasons, is best accomplished by blowing a fuse that is in series
with the switch.
Modem electrical power controls use solid state switching devices
such as Silicon Control Rectifiers, Power Transistors, Solid State
Relays and Triacs. Edwin Mills U.S. Pat. No. 4,315,141 uses a pair
of solid state switches biased by a temperature sensitive
capacitive element as a temperature overload circuit for
conventional electric blankets. In these control systems, a small
signal controls switching of larger load currents. Integrated
circuits or micro processors can be used to provide the control
signal required to operate high speed solid state switching. Micro
circuits of this type are capable of operating at speeds many times
the 50 or 60 HZ. commonly used in AC electrical power supplies.
This capability makes it possible to control each AC cycle. In
fact, the switching can occur as the AC waveform crosses zero. This
type of control can lower the noise generation associated with AC
switching and makes the most efficient use of AC power.
Recent advances in microwatt power control has improved the
reliability of Integrated Circuits by assuring the proper voltage
input to the micro controller. Jamieson and Weiss U.S. Pat. No.
teach an extremly low power voltage detection and switching circuit
to provide power input to an Integrated Circuit "IC" within a
narrow voltage band when only a low power and variable supply is
available. Watchdog timing circuits can be incorporated within an
IC to perform the task of periodically resetting the IC and to
avoid a prolonged lockup or ambiguous operation resulting from
power faults and voltage spikes often associated with AC power.
The Jamieson and Weiss patent, above referred to, is the patent
granted on allowed U.S. patent application Ser. No. 07/655,217,
Filed Feb. 12, 1992.
SUMMARY OF THE INVENTION
The present invention utilizes art electrical feed back circuit and
a semiconductor switching system to control power to a heating
element of the Positive Temperature Co-efficient "PTC" type that
requires a safe operation condition in the event of an open or
short circuit. An integrated circuit is used to signal a solid
state switch to time the on and off proportion of the a.c. electric
power to a flexible PTC heating element in order to control the
temperature. Since characteristically, the PTC element has the
property of increased electrical resistance with increased
temperature the natural effect of increasing temperature is to
throttle down and limit the current draw. The ability to control
the temperature of the heater, by current control or time
proportioned power control is improved. The power control level is
affected externally by a heat scale setting via up-down key pad or
rotary potentiometer and internally by the feedback safety
circuit.
In the first embodiment each of the heater return conductors closes
a current loop through a voltage divider. The voltage of the
junction of the two resistors comprising each of the voltage
dividers is very sensitive to the effect of a break or open circuit
in the heater conductors. The junction voltages are reconditioned
and combined to provide a single voltage that is input into the
control IC. The voltage level is detected and affects the sequence
of micro electronic logic that determines the duty cycle of the
control of power to the heater. If the voltage level is low then
the logic would be caused to stop the activation of the solid state
power switch. In respective versions of a second embodiment one or
both of the heater return conductors closes a current loop clamped
to ground and to a steady D.C. voltage to ground that is supplied
to the control IC and the voltages at the ends of the heater return
conductors are separately supplied to the IC, which then detects
whether there is a fault.
In a preferred embodiment of the safety circuit, the power circuit
of the heating pad contains a fuse that is sized for the normal
current draw. The start up cycle, controlled by the IC, is designed
to provide a limited duty cycle within the first few seconds and to
raise the PTC material temperature and thus raise the electrical
resistance in order to reduce the average inrush current. In this
way the fuse, a slow blow type, avoids the design restraint
associated with high inrush current typical with the PTC heating
elements. With the fuse rated close to the normal current draw a
current surge resulting from a short circuit quickly disables the
power. After the initial cycle, designed to reduce the inrush
current, a preheat cycle starts that raises the temperature of the
pad to a certain temperature by allowing a 100% duty cycle for a
period of time corresponding to the initial setting.
This preferred method of control allows the arrival to the desired
temperature in a minimum time period without subjecting the entire
power and control system to high inrush currents.
In a further development of the invention, the possibility that a
Triac heater switch might lock up in the ON position is
counteracted by adding a second Triac in series, also controlled by
IC.
The response to a fault is so quick that less than a quarter of a
second is an ample allowance for detecting a fault. In consequence,
the first response, or the first fault responses to a fault may be
allowed to interupt the ON condition of the heater switch for a
period no longer than a second, so that a second response will come
quickly if the fault persists, before a final switch-off is
provided.
Instead of the second triac being in series with the heater switch
triac, the second triac may be connected to blow the fuse by
grounding its connection with the heater after enough fault
detections (for example 10 of them or, more generally from 3 to 16
of them) to make it clear that the heater switch triac is locked in
its "on" position.
Many features of one illustrated embodiment can be incorporated in
another illustrated embodiment, as, for example, the different ways
of using a second triac above mentioned.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a safety circuit in the according to the
invention.
FIG. 2 is a block diagram of the heat control and display systems
to which the safety circuit of the invention provides its
output.
FIG. 3 is a diagram of an alternate safety circuit embodiment
showing a redundant safety switch.
FIG. 4 is a circuit diagram if a first version of the preferred
embodiment with two safety circuit inputs to the IC.
FIG. 5 is a block diagram of the microcontroller IC 27 of FIG.
4.
FIG. 6 is a plot of the power, and control signals of the preferred
embodiment.
FIG. 7 is a plot of the feedback signals of the preferred
embodiment
FIG. 8 is a flow chart of the program of the microprocessor shown
in FIG. 5.
FIG. 9 is a circuit diagram of a second version of the preferred
embodiment in which one input to the IC 27 has a D.C.
characteristic and the other has a pulsed signal.
FIG. 10 is a flow chart of the program of the microprocessor shown
in FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a first embodiment of the safety feedback of the
invention circuit. The PTC heater element (not shown) and the power
input to the heater 110 volt 60 HZ, connects to the heater
conductor 1 with the fuse 5 in series relation to the input power.
The circuit is completed by the PTC heater element second power
conductor 2 connected through the Triac T1 to ground. The Triac
will conduct no power until a signal is sent to the gate 12 by the
Intergrated Circuit, IC. To avoid noise associated with switching
AC loads that may affect other appliances, TV's, radios, etc., a
high impediance AC signal is input to the IC through Resistor R9
and clamped to a DC power input voltage through Diode D5. This
signal is used to co-ordinate the firing of the Triac T1 as the AC
power wave form is near the zero crossing. In this way switching
occurs at instantaneous low voltage preventing voltage spikes as
may occur when switching at other than at 0.degree. or 180.degree.
phase angles. The resistance of the PTC heater is between the
conductors attached to 1 and 2. This resistance is low at first
causing a high current draw. As the temperature of the PTC material
heats up the resistance between the two conductors increases and
less current is drawn. The heater is considered to be a parallel
relation between conductors 1 and 2. Conductor 1 is returned to the
control circuit at junction 3 and conductor 2 is returned to the
control circuit at junction 4. The return of conductor 1 through
junction 3 is connected to the conductor 2 through a pair of
resistors R2 and R1 forming a voltage divider at junction 6.
Similarly, the return conductor through junction 4 is connected to
conductor 1 through a pair of resistors R3 and R4. As with R2 and
R1, R3 and R4 form a voltage divider. The junction voltage at 6 and
7 are sensitive to breaks in the respective feed conductor.
Typical values of the components used to demonstrate the action of
this embodiment are listed in table 1. The actual values used will
depend on the required response time to determine a fault.
TABLE 1 ______________________________________ Component Type or
Value ______________________________________ R1 Resistor 10K OHM
1/4 Watt R2 Resistor 100K OHM 1/4 Watt R3 Resistor 1K OHM 1/4 Watt
R4 Resistor 200K OHM 1/4 Watt R5 Resistor 100K OHM 1/4 Watt R6
Resistor 200K OHM 1/4 Watt R7 Resistor 620K OHM 1/4 Watt R8
Resistor 51K OHM 1/4 Watt R9 Resistor 1M OHM 1/4 Watt R10 Resistor
5.1K OHM 1/4 Watt D1-D5 IN 4001 1/4 Watt C1 Capacitor .47 MF 50 V
C2 Capacitor .2 MF 200 V Q1 PNP TRANSISTOR 733 T1 TRIAC 6 AMP
______________________________________
When 110 VAC power is applied to the heater through conductors 1
and 2, the voltage at the return junction 3 is nearly 110 VAC,
assuming the resistance value of the conductor is significantly
less than R1 and R2. For the example of the preferred embodiment,
the conductor resistance is 7 OHMS, R1 is 10,000 OHMS and R2 is
100,000 OHMS. With the resistance values of R1 and R2 shown to be
in a 10 to 1 ratio, the voltage at junction 6 is about 10 VAC. This
voltage half wave rectified through D1 results in a zero to five
volt change at 8. More generally the ratio may conveniently be from
8 to 1 up to 12 to 1.
Storage capacitor C1 holds the voltage at 8 near five volts and a
resistor R5 provides a current path to ground to drop the voltage
when the input voltage is not provided by the voltage divider
R1-R2. The value for C1 and R5 are chosen to stabilize the voltage
at 8 to near 5 volts and to drop the voltage within a specified
time period when the power from the voltage dividers no longer
drives the circuit. For the illustrated embodiment the time
constant, the time for the capacities to drop to 37% of the
voltage, is calculated by the formula TC=R5*C1 or 0.047 seconds.
More generally, from 0.04 to 0.06 seconds is suitable for that time
constant. This time constant is important in determining the length
of time required for the circuit to react to a fault and also the
time that passes after the initial power up before the IC can look
for the fault. The Safety Circuit input to the IC 14 is at junction
9. With the silicon diode D3 between the IC input 9 and junction 8,
a voltage drop of 0.6 volts is expected, therefore, with no break
in conductor 1 or 2 a voltage of 5 to 5.6 exists at the IC Safety
Circuit input.
For detection of a break in conductor 2, a voltage divider is set
up at the return of conductor 2 between R3 and R4. The voltage at
junction 7 is sensitive to a break in conductor 2. Junction 7
voltage is held near 1 volts to ground when conductor 2 is
continuous by the ratio of resistance values of R3 to R4. When
conductor 2 opens as a result of a break, the path to ground now is
through R6 and C2, the junction 7 voltage goes high. Resistor R6
and capacitor C2 form a circuit having a time constant given by the
formula R6*C2.
The signal is halfwave rectified through D2 and current limited by
R8 biasing Transistor Q1 and thus blocking the conduction between
the emitter and collector. The voltage at the collector is then
drained through R16 and the voltage at 11 declines pulling the
safety circuit input voltage low through the forward conduction of
Diode D4. In this manner D3 and D4 act as a logical AND gate, the
junction 9 will go low, less than 1.5 volts, if junction 8 or
junction 11 goes low. The IC will not take protective action if
both diodes are at or above 5 volts. The time required to disable
the control when conductor 2 breaks is the result of charging C2
through R4 and D2. Upon initial startup the voltage at 7 is high,
about 55 volts until the Triac fires connecting the junction to
ground through R3. At this time, it will take 5 time constants of
the curcuit comprising the resistor R6 and the capacitor C2 for R6
to drop the voltage at 10 before Q1 conducts, for the values shown
this would be 100 miliseconds, the waiting period before safe
operations can be determined.
Referring to FIG. 2, the IC 14, used for the control of power
switching by sending a signal to the Triac T1, is powered by a
nominal 5 volt power supply shown as 5.6 volts. More generally that
power supply has a stabilized voltage of from 4 to 7 volts. The
power setting is input by using momentary switches 15, 16, and 17
for on-off, up and down control. The setting status is displayed by
a liquid crystal display 18. In addition to displaying the power or
heat setting, the display can indicate an abnormal operating mode
by flashing or activating a segment of the display that instructs
the user to discontinue use. Similarly an audible alarm can be used
to alert the user.
The IC 14 function used to control the temperature of the PTC
heater is by time-proportioning the power. For a low temperature
setting, for example, the on time may be 2 seconds with a 28 second
dwell or off time in a 30 second cycle. The middle setting would
have a 15 second on time and a 15 second off time. The highest
setting accordingly would provide a 30 second on time or continuous
heating. The lowest setting, in this case 2 seconds on, needs to be
designed with consideration to the maximum time constants that
determine the reaction time of the circuit. The minimum on time may
be as low as 1 second and the cycle can conveniently be from a few
seconds to a few minutes.
The control of the triac (FIGS. 1 and 3) through its control
connection 12 is produced by sequences of unidirectional current
pulses respectively bridging zero-crossing instants of the AC wave
form, resulting in continuous conduction of the AC wave form,
resulting in continuous conduction of the AC through the triac so
long as the sequence of control pulses is not interrupted. As noted
below, in response to the safety circuit the control pulses,
instead of being stopped entirely, may be interrupted for 800
milliseconds only.
In the first embodiment as described previously two seconds is at
least ten times the maximum dwell time before the IC is able to
detect a fault. Once a fault condition is detected, the signal to
fire the Triac is delayed. After the initial fault is detected, a
Triac delay time of 800 miliseconds passes and another signal is
sent to the Triac requiring 200 miliseconds and if another fault is
detected the Triac is again delayed. At this time, a third fault
test sequence can be enacted or the Triac signal can be disabled
for the entire operation. More generally those periods can be
varied safely by at least 25% from the example just mentioned.
To further assure the detection of the fault 200 miliseconds after
the next cycle begins the IC reads the Safety Circuit input signal
and if the fault is detected, the triggering of the Triac is again
by-passed and the drive for the LCD display is switched into a
blinking mode. To avoid further hazardous use especially in an
unattended situaton, a repetitive fault occurrence would cause the
unit to turn off. Repetitive interaction by the user to turn off
and turn on would only cause the display to blink since the fault
condition can be stored in memory. At this time, the only way power
would go to the faulty heater is if the user disconnected the power
cord from the receptacle and again inserted the power plug into the
receptacle. By repowering the controller, the first 200 miliseconds
of heating would again look for the fault and the detection cycle
would repeat again disabling the power to the heater.
The IC function can also improve the reliablity of the disconnect
feature in the event of a short circuit that would result from the
contact of conductors 1 and 2. Referring again to FIG. 1, the fuse
5 may be sized for lower currents and shorter time and avoid the
extreme inrush currents, up to seven times the operating currents,
by pulsing the power to the heating element in the first few
seconds. For example, by only allowing a 10% to 30% duty cycle in
the first five to ten seconds the continuous current would then be
1 ampere instead of 5 amperes when a cold element is first heated.
After the first few critical seconds, a preheat cycle of continuous
operation can be enacted to quickly raise the temperature to the
desired setting.
In a second embodiment redundant Safety Control can be achieved by
including a second switching device in series with the Triac. FIG.
3 shows this combination where even if a Triac fails in the closed
position complete control is achieved by the second Triac T2. Both
switching devices are simultaneously fired by either the same
signal or respectively by separate signals sent by the IC 26. The
Safety Circuit is composed of discrete components, diodes
resistors, capacitors and a transistor operating off low current.
It may be beneficial to use diodes having higher power capabilites
such as IN 4001 instead of the small signal switching type that is
commonly used to handle the loads associated with this type of
circuit. In design careful consideration of component tolerances is
important, especially relating to the capacitors and resistors that
determine the charging and discharging time.
A first version of the preferred embodiment of the safety circuit
invention is shown in FIG. 4. The principal of this preferred
embodiment is based on a dual signal input to the IC and a
comparitave analysis of the two signals to determine if safe
operation is to continue. Unlike the circuits of FIG. 1 and FIG. 3
there is no dependance on time constants and the time to detect a
fault is nearly instantaneous, in fact within one half AC cycle if
necessary. The 110 VAC return signal at pin 3 connects to junction
25 through a current limiting resistor R10 having a resistance of 1
Meg OHM. The junction 25 voltage is clamped to the dc supply
voltage by diode D6. The junction 25 signal, a 60 Hz pulse is input
to the IC at 22. If the conductor between 1 and 3 breaks then R11
rapidly provides a current path to ground and the signal to the IC
at 22 is then ground. Resistor R11 is on the order of 1 Meg OHMS.
The IC is therefore expecting a 5 volt 60 Hz pulse at the input 22
to continue the firing of the Triac. A second signal directly
connected to the ground return at pin 4 is input to the IC at 23.
Connected between the ground return at 4 and the AC 110 v power
input is a 1 Meg OHM resistor R12.
With the ground wire continuous within the heater, between pin 2
and 4, the signal to the IC at 23 is ground. If a break occurs in
the ground conductor then the signal from 110 VAC through the
current limiter R12 is clamped to the 5.6 volt DC input by Diode
D10 for AC Phase angles 0.degree. to 180.degree. and is clamped to
ground through D6 for the AC half wave between 180.degree. and
360.degree.. The IC input at 23 is expected to be ground, if a 60
Hz pulse is detected then a fault in the heater ground conductor
has occured.
The micro controller IC, FIG. 5 includes Read Only Memory "ROM" 29
where the algorithims and instruction set that comprise the program
to control the heating and the display are stored. The instructions
from ROM are processed within the Arithmetic Logical Unit "ALU" 30
and the resulting values are decoded and stored in a data register
as Randam Access Memory "RAM" 31 to be used as input to the
program. The input signals, AC in, safety circuit inputs 22 and 23,
and the control inputs 15, 16 and 17, are received through the data
bus 32. The program determines when power is to be supplied based
on the input from the safety circuit and the control status. The
Triac firing is coordinated with the AC wave form input to the IC
through the AC in port 20 to trigger the Triac at the zero
crossing. A program counter "PC" 32 is required to keep track of
the program steps and index to the next program instruction.
A timing circuit 21 serves to control the clock speed at which the
program operates is made up of a typical of an RC oscillator. A
crystal oscillator can also be used. Typically the clock speed is
in the order of 1 to 2 million cycles per second. The watchdog
timer is set to overload periodically which initiates a device
reset, upon reset the program is initialized and starts from the
beginning. The watchdog timer 28 intermittently times out the
microprocessor operation for a preset period, adjustable between
0.01 and 3 seconds. The time counter 33 and program counter 32 are
also reset. If a lock up occurs, the watchdog timer, having its own
internal oscillator, will continue to countdown and then reset the
program. The timeout mode is also enacted upon power up to assure
the proper voltage is input to the microprocessor, thus allowing
the power circuit time to stabilize. The watchdog timer is
important to guarantee the processing of the safety circuit signal.
It may also reset the microprocessor in all situations involving
noise pulses that may corrupt memory or cause a lock-up. While in
the heating cycle, the IC produces an output signal at port 12 that
triggers the Triac AC connecting to the heater. The output signal
12 controls the firing of the triac. OKI Co. device number MSM64162
is one example of a micro controller IC that can perform the
functions as stated above.
The AC Power input and the Triac Trigger Signal for the embodiment
of FIG. 4 is shown in FIG. 6, the signal time period is 60 Hz 10
cycles. For the same time frame, FIG. 7 shows the possible
combination of signals that will be input to the IC for
determination of safe operation. Referring to FIG. 7A the 60 Hz
pulse at pin 22 and ground at 23 FIG. 7A is the signal combination
required for safe operation. FIG. 7B shows the signals at 22 and 23
when a break in the heater ground conductor has occured. FIG. 7C is
the signal combination resulting from a break in the heater 110 v
conductor. The feedback signals of FIG. 7D would be expected when
both the 110 VAC and the ground heater conductors are open, this
typically occurs if the heater is not connected to the controller.
The signal analysis of groups FIG. 7B and 7C and 7D would result in
the interruption of the Triac Trigger Signal 12, shown in FIG. 6
and thus the interruption of the 110 VAC power to the heater. In
the case of 7B and 7C, this power interruption will prevent the PTC
material from arcing and causing a fire, for the open circuit
condition of 7D when the user has not yet plugged in the heater,
the power interruption eliminates the possibility of electric shock
from touching the plug or receptical.
The program, stored in the ROM section 29 of the IC 27, has a
routine to analyze the feedback signals 22 and 23. Referring to the
routine flow chart of FIG. 8 the first instruction looks for the
safe signal, FIG. 7A. If the pulse is detected at port 22 and
ground is detected at 23 then the result at the first stage is Yes
and the next instruction is to verify that the heating cycle is on.
If the Triac has failed in the short circuit condition and heating
is in the off mode then a No answer to the heating status
indication routs the program to the Heat Status Counter "HSC" sub
routine. The HSC sub routine adds one to the HSC value then
compares the value to 10. If the count is over 10 then 10
consecutive cycles indicate Triac Failure and the sub routine is
routed to fault protection and alarm routine. If the Heating Status
Indicator is on then normal operation is occurring and the Error
Counter and the Heating Status Counter are set to zero and the
routine goes back to the main program. At the first stage if the
pulse is not detected at 22 or a pulse exists at 23 then the answer
is No and if the Heating Status is on, then an error condition
exists that would indicate an unsafe operating condition, at this
point the error counter is indexed by one and in ten cycles,
approximately 87 miliseconds, the subroutine is routed to the fault
safety routine disabling the Triac, flashing the display, flashing
an indicator light or sounding an alarm. The error count is set at
10 for example to react to a fault in 87 miliseconds, the count can
be smaller if a quicker reaction is required. The count should not
be as small as one in order to prevent nuisance failures that may
result from power fluctuation.
A second version of the preferred embodiment with a dual signal
safety circuit feedback is shown in FIG. 9. The 110 V return
circuit is the same as described in FIG. 4 producing a 60 cycle
pulse for input to the IC at port 22. The ground return from pin 4
is connected to the junction of a voltage divides between ground
and the 5.6 volt DC supply. When the ground conductor through the
heater is continuous then this conductor shorts the junction to
ground and the second feedback signal to the IC at 23 is ground,
the same condition as the circuit of FIG. 4. The safe operating
signal configuration is the same as FIG. 7A. In the event of a
broken or open ground heater conductor then the junction between
resistors R13 and R14 is no longer shorted to ground, the voltage
divider becomes active and the junction voltage is 5 volts. The
error condition for the signal input to port 23 is now 5 volts (in
pulses) and the safe condition is ground potential, as in the
previous example of FIG. 4. When arcing occurs, resulting from a
conductor break, the high local temperature of the arc or spark
will cause the PTC material to burn and form carbon in the area
surrounding the arc. The carbon path created by the arcing
condition can conduct current that will produce a lower level AC
signal at 3. To prevent this lower voltage signal from generating
the pulsed signal at junction 25 and to prevent input to the IC at
22, a voltage breakover device 35 is placed in series with the 110
VAC return conductor between 3 and R10. Resistor R15 provides a
current path to ground and is sized to allow the current and
voltage across 35 to be just over its threshold voltage when the
input AC voltage is at the lowest rated voltage, as for example 100
volts. The breakover device 35 shown in this embodiment is a
mineature neon bulb having a minimum breakdown voltage, the voltage
to turn on, of 80 volts. Xenell Co. in Oklahoma, part #A1E, can be
used for this purpose. With the current limiting resistor R15
attached to the ground return the lamp 35 is only on when the Triac
T1 is firing and will act as a heat indicator light, if the current
limiting resistor were attached to the common ground the lamp would
be on during the off cycle as well as the on cycle and can have the
dual function of providing backlighting for the display.
A second Triac T2 can be used to disable (the controller
permanently) in the event of the Control Triac T1 failing in the
short circuit condition. The circuit for the control disablement is
of the common crowbar type that for a short duration connects
ground across the fuse 5, a current limiting resistor "not shown"
can be used to prevent the surge current from causing damage to the
internal wiring of a house or the firing sequence for Triac T2 can
be time proportioned to limit current and accomplish the blowing of
the fuse 5 within a specific period of time, for example, 200
miliseconds. The program routine, FIG. 10, is the same as FIG. 8
for the sequence stages 36 through 48. Stage 49 compares the Heat
Status Counter to 10 as in stage 47 only this time if it is over 10
then the next program sequence is the Triac 2 firing routine that
disables the unit. If at stage 49, the HSC is not greater than 10,
then the error condition is not a result of the Triac 1 T1 failing
closed and the routine is directed to the stage 50 that will flash
the display after disabling the Triac 1 signal.
The circuits of FIGS. 1 through 10 illustrate the principles of the
invention and it will accordingly be recognized that the
application of the principles of the invention variations and
modifications are possible in many respects.
By application of the principles of the invention discribed herein,
other applications will become apparent to those skilled in the
art. No limitations are intended or implied herein, other than
those of the appended claims.
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